#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
public:
    int takeAttendance(vector<int>& records) {
        if (records[0] != 0)
            return 0;
        int left = 0, right = records.size() - 1;
        while (left < right) {
            int mid = left + (right - left + 1) / 2;
            if (records[mid] == mid)
                left = mid;
            else
                right = mid - 1;
        }

        return right + 1;
    }
};